Plate-fin heat exchangers have particular applications in cryogenic plants that are used in natural gas processing and an air separation. Such heat exchangers are typically fabricated by fusing layers of aluminum flow passages having interior fining within a vacuum brazing oven. In a typical brazing operation, fins, parting sheets and end bars are stacked to form a core matrix. The core matrix is placed in the vacuum brazing oven where it is heated to the brazing temperature in a clean vacuum environment.
Small air separation plants, typically less than 400 tons per day oxygen utilize a plate-fin heat exchanger that has a single core matrix. However, for higher flows of heat exchange duty, the plate-fin heat exchanger is constructed from several of such cores that can be connected in series or in parallel.
Heat exchanger efficiency design is limited by the fact that each heat exchanger must be formed from individually brazed cores, which are in turn constrained in maximum cross-sectional flow areas because the brazing ovens are limited in size. Typically, brazing ovens have a length of between about 6.5 meters and about 10.0 meters and a width of between about 1.0 meters and about 2.0 meters. Consequently, the size and the design of a plate-fin heat exchanger is limited by the size of the furnace.
Typically, inlets and outlets for the layers contained within the plate-fin heat exchanger are positioned at opposite ends of the plate-fin heat exchanger. For a given heat transfer duty, the more compact the design of the plate-fin heat exchanger, the greater the fin density in order to provide an effective heat exchange area. For a given volume of the plate-fin heat exchanger, fin density can be increased to increase the effective heat exchange area. Fin density is defined as the number of individual fins extending from the top to the bottom of a flow passage of a layer per inch of flow width. Obviously using higher fin density will result in a higher heat transfer surface area per unit volume. The increase in surface area inevitably comes at the expense of more frictional pressure drop. It is also possible to increase the rate of heat transfer within a given heat exchanger by the use of fins which interrupt the flow or otherwise add turbulence to the fluid passing through the layer. In fact straight “plate fins” are rarely used as the primary heat transfer fin. Many such fin designs are available from plate-fin heat exchanger manufacturers—examples being wavy fins, perforated fins and serrated fins. All of these designs will provide higher rates of heat transfer at the expense of increased pressure drop.
In certain applications, such as air separation, pressure drop is a critical design consideration. In air separation, air is compressed and purified and thereafter the air is cooled to near its dew point prior to its introduction into a distillation column. The product and waste streams produced by the distillation column flow back through the heat exchanger to cool the incoming air. Obviously, if the fin density were increased and therefore the pressure drop, the air would have to be compressed at a much higher pressure to overcome the increase in pressure drop in both the incoming air stream and the product and waste streams. While the degree of increase in compression that may be required to overcome such increase pressure drop for the incoming air stream is not particularly critical, the amount of increase of the compression pressure of the incoming air required to overcome increased pressure drops for the product and waste streams can result in excessive power consumption.
In order to decrease the pressure drop for a particular plate-fin heat exchanger, it is known to increase the cross-sectional flow area of each of the layers. One known way to increase the cross-sectional flow area is to utilize inlets and outlets for the flow along the length of the plate-fin heat exchanger (i.e. the longest dimension of the plates from which the heat exchanger is formed) so that the liquid flows parallel to the width of the heat exchanger. Hence, the entire length of the heat exchanger forms part of the cross-sectional flow area to obtain a maximum increase in the flow area. In French patent application 2844040, the incoming air of an air separation plant that also produces nitrogen and oxygen is subjected to indirect heat exchange with the nitrogen and oxygen at alternating layers in which the inlet and outlets for these components are situated along the length of the heat-exchanger. The disadvantage of such a design is that the flow of each component must be distributed and redistributed across the length of the heat exchanger and such redistribution causes the flow to change direction and therefore incur a pressure drop. Additionally, since the flow is parallel with the width and as indicated above, the size of a plate-fin heat exchanger is limited by the size of the brazing, the fin density has to be increased to a level that is sufficient to obtain the required heat exchange duty for the heat exchanger. Thus, there does not exist a lot of flexibility in the design of such a heat exchanger.
As will be discussed hereinafter, the present invention provides a plate-fin heat exchanger in which the cross-sectional flow area is increased over a plate-fin heat exchanger of the prior art and that inherently possesses a great degree of design flexibility and application. Further advantages will become apparent from the following discussion.